1. Field of the Invention
The present invention is addressed to improved waveguide holograms and particularly multimode waveguides having superior input light coupling.
2. Discussion of the Background
Waveguide holography offers many advantages when compared to conventional holograms. Waveguide holograms provide for the recording and reconstructing of holographic images with lightwaves which propagate along optical waveguides. When this is contrasted with conventional holograms, a higher image-to-background contrast is obtained and a higher global diffraction efficiency is obtained with low diffraction efficiency materials. Furthermore, waveguide holograms provide minimized illumination space and obstruction free viewing.
A waveguide hologram is strictly defined as a hologram whose image wavefront is reconstructed with a guided light from the waveguide. Thus, a waveguide hologram (WGH) consists of an input coupler 10, the waveguide itself 20 and the holographic emulsion 30 as shown in FIG. 1. A source of light 15 is coupled into the waveguide and this waveguide is normally a sheet of transparent material with two surfaces which are locally parallel and optically polished. The refractive index of a waveguide must be higher than the index of the environment in order to achieve the principles of waveguide transportation.
The different types of waveguides are distinguished by the size of the dielectric which constitutes the waveguide and by the mode of illumination. Prior art devices in the waveguide hologram field utilize edge illumination in a single mode waveguide or in a multimode waveguide. Single mode waveguides are used to couple integrated circuits with optics in interconnected electronic packages. These single mode waveguides are very clean, however, they require an extremely precise orientation of the input light source. That is, these type of WGHs are very thin and the light must be coupled at the edge very carefully to provide proper alignment.
Another type of waveguide is a multimode waveguide which involves internal reflections.
The prior art edge lit multimode waveguides suffer from problems with coupling efficiencies and a requirement for input light direction. Furthermore, the matching of the size of the input light i.e. the diameter, is an important factor in these edge lit multimode waveguides as they are in the edge lit single mode waveguides. The categories of waveguide based upon the width (w) of an incident beam of light, the optical waveguide thickness (t) and the wavelength of the incident light wave (.lambda.) are shown in FIG. 2a-c.
A first category of thin film waveguide as shown in FIG. 2a has .lambda..about.t&lt;&lt;w. This is a thin guided layer coated on a glass substrate of the type used in integrated optics. The major drawback to this type of structure of course is that it is difficult to achieve high coupling efficiency and white light coupling is impossible.
A second category, as shown in FIG. 2b, has a thick substrate waveguide wherein .lambda.&lt;&lt;t.about.w. Although light coupling in the edge is easy, such light coupling creates multiple discrete "bounces" at the waveguide surfaces and as a result, edge lighting can only provide discrete holograms.
In the third category of FIG. 2c there is a dielectric block wherein t&gt;&gt;w&gt;&gt;.lambda.. This type of structure allows white light to be edge-introduced and illuminates a hologram with nobounce, however, it is much too bulky to be used and to be of interest in the field of waveguide holograms.
These types of edge lit multimode waveguides have the above associated disadvantages and it is the purpose of the present invention to provide an improved waveguide structure which eliminates these disadvantages and provides ease of construction.